1. Field of the Invention
The present invention pertains, in general, to matte biaxially oriented polypropylene films having improved matte property and processability. More specifically, The present invention is directed to a matte biaxially oriented polypropylene film having low gloss (45xc2x0,  less than 10%) and high haze ( greater than 70%), which solves problems related to increase of silver (or bright) spots and roughness of a matte pattern, and a shortened die cleaning cycle due to die build-up.
2. Description of the Prior Art
Generally, films, made from polyolefin resin balanced in terms of price and performance, are excellent in mechanical, thermal and electrical properties, and have superior chemical resistance and formability. Thus, such films have been mainly used for packaging of foods and goods, and for coating of paper and other materials, or as lamination materials. Among the films, a matte film which is able to give the film smooth feelings due to decrease of surface gloss is evaluated as a highly valuable product increasing marketability of packaging, coating or lamination materials, and has been widely used. In particular, a matte biaxially oriented film consisting mainly of polypropylene resin constitutes most polyolefin-based matte films, and has been variously used for food packaging and paper lamination.
The matte function is based on scattered reflection of light by uneven reflecting surface. The methods for providing the matte property to the film surface are classified into: (1) surface treatment such as coating and etching of the surface of a conventional film, (2) use of the difference of flowability, shrinkage and refractive index in an incompatible resin blend, (3) formation of uneven film surface using an inorganic filler and (4) embossing treatment for formation of a cooling roll having an uneven surface. In the matte biaxially oriented polypropylene film, the method described in the above (2) is mainly used. As the incompatible resin, a resin blend of polyethylene (high viscosity/low melt index) and polypropylene (low viscosity/high melt index), having a large difference in viscosity, is typically used.
In the method for providing the matte property by use of the incompatible resin blend having a large difference in flowability, as the degree of incompatibility and difference of flowability between resins in the blend becomes larger, or as a resin composition in a bi-component system approaches 50/50 wt %, the matting effect is increased. However, if the difference of flowability between the incompatible resins is too large, poor mixing and numerous fish eyes are generated upon extrusion due to large difference of viscosities. Poor mixing increases roughness of the matte pattern and fish eyes generate silver (or bright) spots of the matte film during stretching. The term xe2x80x9csilver spotxe2x80x9d refers to a slightly lighter portion on the film surface, while the term xe2x80x9cbright spotxe2x80x9d refers to a glittering portion of homo polypropylene core exposed by non-wetting of the matte resin to the skin. The silver (or bright) spots are generated due to the poor stretchability and weak interface of fish eye, tearing of interface during stretching. Such phenomena are undesirable for food packaging and paper lamination film, and decrease marketability of the materials.
In addition, when the incompatible resin blend having large difference in flowability is used to increase the matting effect, problems such as die build-up are caused. As for the die build-up, the incompatible resins flow non-uniformly owing to different flowability thereof and are deposited onto the die like impurities. Very low molecular weight components of resin blend also tend to be deposited onto the die. If the die build-up is excessive, the machine should be shut down to clean the die.
Further, the silver (or bright) spot of the matte film, roughness of the matte pattern and the die build-up are influenced by a molecular weight distribution (MWD) of the used resin. Typically, the matte property improves as the molecular weight distribution increases. However, if the MWD is too broad, increase of the silver (or bright) spots of the matte film and roughness of the matte pattern, due to high molecular weight portion, and increase of the die build-up, due to low molecular weight portion, become extreme. In particular, in the case of having a bimodal molecular weight distribution, the above problems are rendered much worse.
Therefore, as the matte biaxially oriented polypropylene film, a film having haze of 60-70% and gloss (45xc2x0) of 10-15% is typically used.
Examples of the conventional techniques disclosed to improve the properties of the matte biaxially oriented film are as follows:
WO 8910839 discloses a biaxially oriented polypropylene film comprising a heat-sealable HDPE (high density polyethylene) as a surface layer, which is advantageous in light of excellent transparency, such as low haze and high gloss, and low friction coefficient.
In U.S. Pat. No. 4,578,316, there is disclosed an oriented film structure comprising a polypropylene film having on at least one surface thereof a layer of a blend of (I) a member selected from the group consisting of a medium density polyethylene, high density polyethylene and mixtures thereof and (II) polypropylene. Such a film has advantages of low friction coefficient, and excellent printability and transparency (low haze and high gloss). As seen from the aforementioned patents, which pertains to the production of films having excellent transparency using polyolefin blend, the matte appearance of the blend not only depends on a kind of the blended polymer resin (polyethylene, polypropylene, etc.) but also a molecular weight and a molecular weight distribution (MWD) of the blended resin.
Furthermore, preparations of a matte surface layer in a biaxially oriented film using a polyolefin resin blend component are disclosed in U.S. Pat. Nos. 5,364,704, 5,366,796, 5,473,016, 5,492,757, 5,494,717, 5,496,600, 5,501,905, 5,516,563, 6,322,894, and EP Publication No. 0 367 613 B1.
In U.S. Pat. Nos. 5,364,704 and 5,366,796, there is disclosed a matte biaxially oriented film having haze value greater than 40%, comprising a resin blend having MI of 1.5-12 g/10 min (230xc2x0 C.) of polyethylene having MI of 0.5-1.5 g/10 min (190xc2x0 C.) and polypropylene having MI of 0.5-10 g/10 min (230xc2x0 C.). But, the above film suffers from the disadvantage of not obtaining sufficient matting effect due to small MI difference between the resins.
In U.S. Pat. No. 5,473,016, there is also disclosed a matte film comprising a resin composition of propylene copolymer (component 1), very linear low density polyethylene (VLLDPE, component 2) and polyethylene resin (component 3) modified with epoxy and hydroxide groups. Optionally, inorganic fillers may be added to the resin composition. However, this film had a drawback, such as high preparation cost due to use of modified polyethylene resin and VLLDPE. Further, the addition of inorganic filler such as CaCO3 to the biaxially oriented film leads to the better matting effect in the quantitative aspect, while the matte layer is peeled off and silver (or bright) spots may be generated in qualitative aspect.
In EP Publication No. 0 367 613 B1, there is also disclosed a matte film using an incompatible resin blend. As the resin, HDPE and polypropylene, having MI of 1 g/10 min or less, are used, and preferably, inorganic fillers such as SiO2 may be added thereto. However, the above film is opaque due to addition of the inorganic filler, and easily has silver (or bright) spot generated upon stretching.
In U.S. Pat. No. 5,492,757, there is also disclosed a film comprising a matte layer consisting of a blend of polyethylene and polypropylene, in which polyethylene having MI of 1-50 g/10 min and polypropylene having MI of 1.5-12 g/10 min are used, and CaCO3 or TiO2 is added to the matte layer or the other layers, thus increasing whiteness and opacity. As for this film having excellent printability, the reduced matte property because of the resin blend having low MI difference used for the matte layer is reinforced with the added inorganic filler. But, because of the inorganic fillers used, silver (or bright) spots may be generated upon stretching and the matte property is not high due to high gloss.
In U.S. Pat. No. 5,494,717, there is also disclosed a multiplayer polypropylene film of high shrinkage having at least one matte surface comprising a mixture or blend of a homopolymer (isotactic homopolypropylene) having MI of 0.5-15 g/10 min (preferably 1.5-6.0 g/10 min) and propylene copolymer (component 1) and HDPE having MI of 0.2-50 g/10 min (ISO 1133 at 50N/190xc2x0 C.), preferably 5-25 g/10 min, or a blend of HDPE and propylene copolymer (component 2). However, such a film is disadvantageous in that MI difference between two components is not large, and the film has inferior matte property owing to haze of 60-70%.
U.S. Pat. No. 5,496,600 discloses a matte twist wrapping film, using components similar to the components disclosed in U.S. Pat. No. 5,494,717 as the matte layer. In this patent, a degraded polypropylene is used for a core layer but the MI difference between the components 1 and 2 used for the matte layer is not large, and thus sufficient matte appearance cannot be exhibited. Also, haze of the above film is maximally 70%.
In U.S. Pat. No. 5,501,905, there is also disclosed a film having a matte layer consisting essentially of a resin composition of polypropylene/polyethylene block copolymer (MI 3-6 g/10 min), ethylene-based copolymer modified with a polar group (MI 3-8 g/10 min), propylene copolymer (MI 3-6 g/10 min), HDPE (MI 1-2 g/10 min) and 5% or less SiO2.
In U.S. Pat. No. 5,516,563, there is also disclosed an opaque, matte film, in which similar components to those disclosed in U.S. Pat. No. 5,494,717 are used for the matte layer, and the degraded polypropylene and the inorganic filler are used for the core layer. The matte layer is formed of a mixture or blend of homopolypropylene and propylene-based copolymer (component 1) and HDPE having MI of 1.0-50 g/10 min(ISO 1133 at 50N/190xc2x0 C.) or a blend of HDPE and propylene-based copolymer (component 2). However, the film has an opaque appearance due to the inorganic filler used for the core layer and optionally used for the skin layer. Also, MI difference of the components 1 and 2 used for the matte layer is not large, and sufficient matte appearance cannot be exhibited. Use of the inorganic filler in the matte layer may cause generation of silver (or bright) spot.
In U.S. Pat. No. 6,322,894, there is also disclosed a matte film having improved cuttability, in which HDPE is used for the core layer and the matte layer is made of propylene homopolymer or propylene copolymer (component 1) and ethylene homopolymer or ethylene copolymer (component 2). In this patent, polydialkylsiloxane is used to decrease die build-up, but the film does not show matte appearance including low gloss and high haze, as can be seen in the Examples of the above patent (gloss: 23-70%, haze: 3.5-41%).
Leading to the present invention, the intensive and thorough research on matte films, carried out by the present inventors aiming to solve the problems encountered in the prior arts, led to development of a matte biaxially oriented film, in which a resin having both broad and continuous molecular weight distribution is used, together with an organic peroxide, whereby problems in the conventional films having low gloss and high haze, such as increase of silver (or bright) spots, roughness of a matte pattern and die build-up, can be solved.
Therefore, it is an object of the present invention to provide a matte biaxially oriented polypropylene film having low gloss (45xc2x0,  less than 10%) and high haze ( greater than 70%) which shows excellent matte property and processability by significantly improving problems encountered in the conventional films, such as increase of silver (or bright) spots, roughness of a matte pattern and die build-up.
In accordance with an embodiment of the present invention, there is provided a matte biaxially oriented polypropylene film, comprising one or more surface layers made of a composition comprising 100 parts by weight of a resin blend of (I) 40-60 wt % of polyethylene and (II) 60-40 wt % of polypropylene, (III) 0.005-0.04 parts by weight of a peroxide and (IV) 0.1-0.5 parts by weight of a fluoroelastomer or a metal stearate,
wherein the component I is composed of a copolymer of ethylene and xcex1-olefin containing 3-8 carbons, the copolymer having density of 0.945 g/cm3 (ASTM D 1505) or more; and the component II is composed of ethylene-propylene, butene-1-polypropylene or ethylene-butene-1-propylene copolymer, the content of ethylene, butene-1 or combination thereof being 1-5 wt % based on the component II.